RESEARCH DESCRIPTION

There are many problems of both fundamental and of practical importance
that require measurement of extremely low concentrations of certain impurities.
Molecular Spectroscopy provides one approach that excels in the high
specificity provided by the detailed structure in the spectrum, particularly
for molecules in the gas phase. Lehmann’s group has been working
on the development of new trace sensors, largely based upon the method
of cavity ring-down spectroscopy (CRDS). In CRDS, one forms a stable
optical cavity using mirrors with reflectivity > 99.99% and observes
absorption of a sample contained inside the cavity by an increase in
the rate of decay of light that is trapped between the mirrors. Sample
absorption as low as 1 part in 109 per pass of the cell can be measured
in this way. The Lehmann group pioneered the use of low cost and rugged
diode lasers developed for the telecom industry in CRDS and has demonstrated
detection of a number of small molecules, such as H2O, NH3,
and CH4 at levels below one part per billion in a sample gas.
Tiger Optics, Inc. is now selling instruments based upon this work to
several industries. We have developed a new, fiber optic version
of CRDS and have demonstrated that this could be used to detect a single
cell that sticks to the surface of an optical fiber. We are presently developing an ultrabroad bandwidth version of CRDS
that will allow multiple chemical species to be monitored simultaneously,
such as with an FTIR, but with much higher sensitivity. The key enabling tchnologies are prism retrorefletors and a supercontinuum lightsource. Breath analysis
for medical diagnosis is an important potential application of CRDS that
we beginning to work in this area. In collaboration with Dr. Ben Gaston of the UVa Medical School, we are developing a mid-IR based nitric oxide detector.
NO in breath detector can serve as a diagnoistic marker for both asthma and infection. We are also developing technology to use near IR CRDS to determine the
13C/12C and D/H isotope ratios of methane in atmospheric air. These give informaton on the sources and sinks of methane in the environment.

Spectroscopy in super fluid Helium

Research in the Lehmann group has long used laser spectroscopy and theoretical
modeling to study molecular dynamics – studying chemical reactions
at their most fundamental level. In recent years, this line of work has
focused on the spectroscopy of atoms and molecules dissolved in nanodroplets
of superfluid helium. Helium Nanodroplet Isolation (HENDI) combines many
of the most attractive features of both high resolution, molecular beam
spectroscopy and more traditional rare gas matrix spectroscopy. The droplets
cool any solvated molecule down to a temperature of only 0.38 K but remain
liquid, which allows molecules to move and rotate nearly freely with
relaxation times three to four orders of magnitude longer than in traditional
liquids. This allows for the study of the interaction of molecules with
a unique solvent of very low entropy and where quantum effects are dominant. Fundamental questions are yet unresolved, such as how
the molecules come into equilibrium with the superfluid and why quantized
vortices (which are common in build liquid helium) have not been observed
in the droplets. The droplets allow the production of new chemical species
and new isomers of known compounds.

We are working on the spectroscopy of free radicals in helium. Traditional
wisdom is that the reaction of two free radicals can occur without a
barrier, but high level ab initio calculations suggest that
in many such reactions (such as O2 + O -> O3),
small entrance channel barriers exist and these are believed to play
an important role in the rates of three body recombination; a process
that produces O3 in the atmosphere. It should be possible
to quench entrance channel complexes and study their properties using
HENDI. We have a "hydrogen cracker" that will produce an effusive beam of atomic hydrogen, which we will dope into the droplets. The plan is to study hydrogen addition reactions, such as H + CO -> HCO, which are believed to proceed at low temperatures (posibly due to tunneling) and are important reactions in interstellar space. We are finishing construction of a new beam machine to study ions in helium droplet. Doped droplets (which have a very low velocity spead) will be mass selected using a hemispherical energy analyzer. This will allow several novel measurements. We plan to study the translational motion of the ions inside the droples and determine the rate of translational cooling as a function of droplet size and the chemical nature of the ion. By exciting the ions with circularly polarized microwaves, we hope to create vortices in the droplets. The binding energies of atomic and molecular cluster ions will be determined from the number of helium atoms evaporated from the droplet after cluster
formation.